Ethylene oligomerization is one of the most important reactions in the industry of olefin polymerization. Low cost small molecular olefins can be converted into high value-added products by oligomerization. Ethylene oligomerization products, i.e., linear alpha olefins (LAO) are important organic chemical raw materials. For example, C4-C30 LAO can be used for preparing household cleaners, flotation agents, emulsifying agents, lubricant components for refrigerants or drilling fluids, plasticizers, various additives, low-viscosity synthetic oils, polymers, copolymers, additives for oil or oil products, higher alkyl amines, higher organoaluminum compounds, higher alkaryl hydrocarbons, higher fatty alcohols and fatty acids, epoxides, additives for heat carriers, and so on. Adhesives, sealants and paint can also be synthesized based on C20-C30 LAO. Recently, with the development of the polyolefin industry, the worldwide demands for alpha olefins grow rapidly, wherein, most alpha olefins are prepared based on ethylene oligomerization.
The catalysts used in ethylene oligomerization mainly include nickel-based, chromium-based, zirconium-based, and aluminum-based catalyst systems, and so on. Recently, the complexes of iron (II) or cobalt (II) with imino-pyridyl tridentate ligands for catalyzing ethylene oligomerization have been reported respectively by Brookhart's group (see Brookhart M et al, J. Am. Chem. Soc., 1998, 120, 7143-7144 and WO99/02472 published in 1999) and Gibson's group (see Gibson V. C. et al, Chem. Commun., 1998, 849-850 and Chem. Eur. J., 2000, 2221-2231), in which both the catalytic activity and selectivity of alpha olefins are high. Therefore, such complexes have a promising prospect in the industrial application. As to such complex catalysts of iron (II) or cobalt (II), the synthesis of ligands is the key point. Whether such complexes can be obtained and the cost thereof are determined by the synthesis method of ligands.
In the prior art, for example, in organic chemistry textbooks Heterocyclic Chemistry—Structure, Reaction, Synthesis and Application (Li Runtao, Ge Zemei, Wang Xin, translation, Chemical Industry Press, 2008, 1 (1): 273) and Fundamental Organic Chemistry (Xing Qiyi, et., Higher Education Press, 2005, 12 (3):917), it both discloses using nitrobenzene as an oxidant to oxide the hydrogen linked with N in nitrogen-containing heterocyclic compounds. However, it does not disclose that when nitrobenzene is used as an oxidant, an acyl at the ortho position of nitrogen can be generated.

Sun Wenhua's group of Institute of Chemistry, Chinese Academy of Sciences (see Sun Wenhua et. al., Journal of Organometallics 25 (2006) 666-677) first adopts 1,10-phenanthroline imine compounds to coordinate with iron (II) so as to obtain tridentate nitrogen imine complexes, which are then used to catalyze ethylene oligomerization. Both the activity and selectivity of such catalyst are high. However, the defects of the preparing methods for such catalyst are that, the synthesis steps for ligands are too complicated, and that 2-acetyl-1,10-phenanthroline is obtained only when highly toxic potassium cyanide is involved in the reaction. In addition, CN101823996A discloses a process for preparing 2,8-diacyl quinoline, wherein selenium dioxide is substantially used in the oxidation reaction so as to generate 2-acyl quinoline. Such method can also be used to form 2-acyl-1,10-phenanthroline. But in the process, selenium dioxide itself is high toxic chemicals of high price and is difficult to be purchased.
Therefore, the development of a process for preparing ethylene oligomerization catalyst with less synthesis steps, simple processes, low raw material cost and free of high toxic substances has been attached great importance.